Nier–Johnson geometry

An arrangement for a double-focusing mass spectrometer in which a deflection of π 2 radians in a radial electrostatic field analyser is followed by a magnetic deflection of π 3 radians. The electrostatic analyser uses a symmetrical object-image arrangement and the magnetic analyser is used asymmetrically.
PAC, 1991, 63, 1541 (Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)) on page 1546
Orange Book, p. 201